ULTRASONIC INSPECTION

1. ULTRASONIC INSPECTION
Ultrasonic inspection is a nondestructive method in which beams of high
frequency sound waves are introduced into materials for the detection of
surface and subsurface flaws in the material.The sound waves travel
through the material with some attendant loss of energy (attenuation) and
are reflected at interfaces.
The reflected beam is displayed and then analyzed to define the presence
and location of flaws or discontinuities. The degree of reflection depends
largely on the physical state of the materials forming the interface and to a
lesser extent on the specific physical properties of the material.

3. For example, sound waves are almost completely reflected at metal/gas
interfaces. Partial reflection occurs at metal/liquid or metal/solid interfaces,
with the specific percentage of reflected energy depending mainly on the ratios
of certain properties of the material on opposing sides of the interface.
Most ultrasonic inspection is done at frequencies between 0.1 and 25 MHz
well above the range of human hearing, which is about 20 Hz to 20 kHz.
Ultrasonic waves are mechanical vibrations; the amplitudes of vibrations
in metal parts being ultrasonically inspected impose stresses well below the
elastic limit, thus preventing permanent effects on the parts.

4. Ultrasonic inspection is one of the most widely used
methods of nondestructive inspection.
Its primary application in the inspection of metals is the
detection and characterization of internal flaws; it is also used to
detect surface flaws, to define bond characteristics, to measure
the thickness and extent of corrosion, and (much less frequently)
to determine physical properties, structure, grain size, and elastic
constants.

5. Most ultrasonic inspection instruments detect
flaws by monitoring one or more of the
following:
· Reflection of sound from interfaces
consisting of material boundaries
or discontinuities within the metal
itself
· Time of transit of a sound wave
through the test piece from the
entrance point at the transducer to
the exit point at the transducer
· Attenuation of sound waves by
absorption and scattering within the
test piece
· Features in the spectral response for
either a transmitted or a
reflected signal

6. General Characteristics of Ultrasonic Waves
1. Ultrasonic waves are mechanical waves (in contrast to, for example, light or x-rays, which are
electromagnetic waves) that consist of oscillations or vibrations of the atomic or molecular
particles of a substance about the equilibrium positions of these particles.
2. Ultrasonic waves behave essentially the same as audible sound waves. They can propagate in
an elastic medium, which can be solid, liquid, or gaseous, but not in a vacuum.
3. In many respects, a beam of ultrasound is similar to a beam of light; both are waves and obey a
general wave equation. Each travels at a characteristic velocity in a given homogeneous
medium a velocity that depends on the properties of the medium, not on the properties of the
wave.
4. Like beams of light, ultrasonic beams are reflected from surfaces, refracted when they cross a
boundary between two substances that have different characteristic sound velocities, and
diffracted at edges or around obstacles.

7. 5.1 Longitudinal waves
General Characteristics of Ultrasonic Waves Contd…
Longitudinal waves, sometimes called compression
waves, are the type of ultrasonic waves most
widely used in the inspection of materials.
Longitudinal waves :Similar to audible sound waves the
only type of wave which can travel through liquid
These waves travel through materials as a series of
alternate compressions and rarefactions in which the
particles transmitting the wave vibrate back and forth in
the direction of travel of the waves.
Fig. Schematic of longitudinal ultrasonic waves. (a) Particle
oscillation and resultant rarefaction and compression.
(b) Amplitude of particle displacement versus distance
of wave travel. The wavelength, is the distance
corresponding to one complete cycle.
5. Based on the mode of particle displacement, ultrasonic waves are classified as longitudinal
waves, transverse waves, surface waves, and Lamb waves.

8. Longitudinal waves

9. 5.2 Transverse waves (shear waves)
Transverse waves (shear waves) are also
extensively used in the ultrasonic inspection of
materials.
–Transverse waves are visualized readily in
terms of vibrations of a rope that is shaken
rhythmically, in which each particle, rather than
vibrating parallel to the direction of wave
motion as in the longitudinal wave, vibrates up
and down in a plane perpendicular to the
direction of propagation.
–Transverse waves generated by passing the
ultrasonic beam through the material at an
angle Usually a plastic wedge is used to couple
the transducer to the material

10. 5.2 Transverse waves (shear waves)